1. Field of the Invention
The present invention relates to an EGR-gas flow rate estimation apparatus, for an internal combustion engine, which estimates the flow-rate of EGR gas flowing from an exhaust circulation pipe to an intake passage of the internal combustion engine.
2. Description of the Related Art
Conventionally, there has been widely known an EGR apparatus which circulates a portion of exhaust gas of an internal combustion engine to an intake passage via an exhaust circulation pipe, in order to reduce the amount of nitrogen oxides (NOx) discharged from the engine. Such an EGR apparatus is applied to both spark-ignition engines and diesel engines. In this case, the flow rate of EGR gas is controlled by changing the opening (effective opening area) of an EGR control valve interposed in the exhaust circulation pipe.
Such an EGR apparatus is designed to estimate the flow rate of EGR gas flowing into the intake passage and measure the flow rate of new air flowing into the intake passage; obtain an EGR ratio, which is the ratio of the EGR gas flow rate to the flow rate of all gases taken in by an engine (i.e., a mixture of new air and EGR gas, and hereinafter, also referred to as “intake air”), on the basis of the estimated EGR gas flow rate and the measured new air flow rate; and control the EGR gas flow rate on the basis of the EGR ratio. Accordingly, accuracy of the estimation of the flow rate of EGR gas flowing into the intake passage is extremely important for engine controls such as control of the EGR gas flow rate. In view of the above, a conventional apparatus is designed to estimate the EGR gas flow rate on the basis of a differential pressure across the EGR control valve (hereinafter referred to as “across differential pressure”) and the opening of the EGR control valve (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-280202 (paragraph 0005 and page 4). The conventional apparatus is designed to estimate the EGR gas flow rate, by using, as the across differential pressure, the difference between a pressure detected by means of a boost sensor (intake pressure sensor) disposed in the intake passage and a pressure detected by means of a pressure sensor disposed at an appropriate position upstream of the EGR control valve. However, in some cases, the differential pressure between points immediately before and after the EGR control valve may greatly differ from the across differential pressure used for estimation of the EGR gas flow rate, stemming from pipe friction produced between the EGR gas and the exhaust circulation pipe. In such a case, the EGR gas flow rate cannot be accurately estimated.
In particular, an error involved in estimation of the EGR gas flow rate caused by the pipe friction increases, and thus, estimation accuracy of the EGR gas flow rate deteriorates, in the case where the flow rate of EGR gas passing through the EGR control valve (accordingly, the flow rate of EGR gas flowing into the intake passage) is estimated by use of a general formula relating the flow rate of a compressible fluid passing through a throttle portion (EGR control valve), in which the flow rate of the compressible fluid is represented on the basis of an upstream pressure of the compressible fluid at a point immediately before the throttle portion and a downstream pressure of the compressible fluid at a point immediately after the throttle portion.